The present invention relates to a process for the anionic polymerization of vinylaromatic monomers or dienes in the presence of a lithium organyl or lithium alcoholate and a magnesium or an aluminum compound.
The anionic polymerization of styrene copolymers or styrene/butadiene copolymers in the presence of alkyllithiums and retardant additives, such as alkylaluminums or alkylmagnesiums, is disclosed, for example, in WO 98/07765. WO 97/33923 describes polymerization initiators which contain, for example, dialkylmagnesium and alkyllithium or lithium alcoholates. These initiators permit the polymerization of styrene polymers also at relatively high monomer concentrations.
WO 99/42498 describes the advantageous use of the mixtures of alkylaluminums and alkylmagensiums as retardant additives.
Furthermore, polymerization initiators which contain sterically hindered phenolates of aluminum (WO 99/42499) or of alkaline earth metals (JP-A-11-116613) were described for the anionic polymerization of styrene.
Styrene monomers which are not used immediately after the distillation for the polymerization must be mixed with stabilizers for storage and transport. Said stabilizers may influence the polymerization rate of the lithium-initiated polymerization, in particular in the presence of said retardant additives. As a rule, stabilizers must therefore be removed from the styrene before the polymerization.
It is an object of the present invention to provide a process for the controlled anionic polymerization of vinylaromatic monomers or dienes, which process can be used in particular for stabilized monomers.
We have found that this object is achieved by a process for the anionic polymerization of vinylaromatic monomers or dienes in the presence of a lithium organyl or lithium alcoholate and a magnesium or an aluminum compound, wherein a sterically hindered phenol or amine is added.
Examples of suitable sterically hindered phenols are alkylated phenols and phenol derivatives, such as 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-nonylphenol, 4,4xe2x80x2-butylidenebis(2-tert-butyl-5-methylphenol), 4,4xe2x80x2-thiobis(2-tert-butyl-5-methylphenol), 2,2xe2x80x2-thiobis(4-methyl-6-tert-butylphenol), 2-methyl-4,6-bis[(octylthio)methyl]phenol, 2,2xe2x80x2-methylenebis(4-methyl-6-tert-butylphenol), 2,2xe2x80x2-methylenebis[4-methyl-6-(1-methylcyclohexyl)phenol], 2,2xe2x80x2-isobutylidenebis(4,6-dimethyl-phenol), 2,4-dimethyl-6-(1-methylcyclohexyl)phenol, 1,1,3-tris(2xe2x80x2-methyl-4xe2x80x2-hydroxy-5xe2x80x2-tert-butylphenyl)butane, styrylated, sterically hindered phenols, 1,3,5-tris[3,5-bis(1,1-dimethylethyl)-4-hydroxybenzyl]-2,4,6-trimethylbenzene, 2,4-dimethyl-6-(1-methylpentadecyl)phenol, 3,3xe2x80x2,3xe2x80x3,5,5xe2x80x2,5xe2x80x3-hexa-tert-butyl-a,axe2x80x2,axe2x80x3-(mesitylene-2,4,6-triyl)-tri-p-cresol, 6,6xe2x80x2-di-tert-butyl-2,2xe2x80x2-thiodi-p-cresol, alkylated hydroquinones, for example 2,5-di-tert-amylhydroquinone, butylated reaction products of p-cresol and dicyclopentadiene (e.g. CAS Reg. No [68610-51-5]) or xcex1-tocopherol.
Examples of suitable amines are secondary, aromatic amines, such as diphenylamine derivatives, for example diphenylamine alkylated with 2,4,4-trimethylpentene, N,N-dimethylindoaniline or the stabilizers known as HALS compounds (hindered amine light stabilizers).
A particularly preferably used sterically hindered phenol is 2,6-di-tert-butyl-4-methylphenol or 4-tert-butylpyrocatechol.
The sterically hindered phenol and/or amine can alternatively be added to the initiator mixture, to the individual initiator components, to the monomers or to the reaction mixture.
Vinylaromatic monomers and/or dienes which already contain a sterically hindered phenol or amine as a stabilizer can also particularly advantageously be used. As a rule, stabilized monomers contain from 1 to 200, preferably from 5 to 50, ppm of the sterically hindered phenol or amine. In this case, a constant amount of sterically hindered phenol can be achieved in the polymerization solution by separate metering in, even in the case of varying stabilizer contents of monomers used. This also makes it possible to use different raw material qualities.
For example, styrene, xcex1-methylstyrene, p-methylstyrene, ethylstyrene, tert-butylstyrene, vinyltoluene, 1,2-diphenylethylene or 1,1-diphenylethylene or mixtures may be used as vinylaromatic monomers. Styrene is particularly preferably used.
The dienes used may be, for example, butadiene, 1,3-pentadiene, 2,3-dimethylbutadiene, isoprene or mixtures thereof.
Usually, mono-, bi- or polyfunctional alkali metal alkyls, aryls or aralkyls are used as anionic polymerization initiators. Organolithium compounds, such as ethyl-, propyl-, isopropyl-, n-butyl-, sec-butyl-, tert-butyl-, phenyl-, diphenylhexyl-, hexamethylenedi-, butadienyl-, isoprenyl- or polystyryllithium, or the polyfunctional 1,4-dilithiobutane, 1,4-dilithio-2-butene or 1,4-dilithiobenzene are expediently used. The required amount of alkali metal organyl depends on the desired molecular weight, the type and amount of the other metal organyls used and the polymerization temperature. As a rule, it is from 0.002 to 5 mol percent, based on the total amount of monomers.
The polymerization can be carried out in the presence or absence of a solvent. The polymerization is expediently effected in an aliphatic, isocyclic or aromatic hydrocarbon or hydrocarbon mixture, such as benzene, toluene, ethylbenzene, xylene, cumene, hexane, heptane, octane or cyclohexane. Solvents having a boiling point above 95xc2x0 C. are preferably used. Toluene is particularly preferably used.
Suitable magnesium compounds are those of the formula R2Mg, where the radicals R independently of one another are each hydrogen, halogen, C1-C20-alkyl or C6-C20-aryl. Dialkylmagnesium compounds, in particular the ethyl, propyl, butyl, hexyl or octyl compounds available as commerical products, are preferably used. The hydrocarbon-soluble (n-butyl)(s-butyl)magnesium or (n-butyl)(n-octyl)magnesium is particularly preferably used.
The aluminum compounds used may be those of the formula R3Al, where the radicals R, independently of one another, are each hydrogen, halogen, C1-C20-alkyl or C6-C20-aryl. Preferred aluminum organyls are the trialkylaluminums, such as triethylaluminum, triisobutylaluminum, tri-n-butylaluminum, triisopropylaluminum or tri-n-hexylaluminum. Triisobutylaluminum is particularly preferably used. Other suitable aluminum organyls are those which are formed by partial or complete hydrolysis, alcoholysis, aminolysis or oxidation of alkyl- or arylaluminum compounds. Examples are diethylaluminum ethoxide, diisobutylaluminum ethoxide, diisobutyl(2,6-di-tert-butyl-4-methylphenoxy)aluminum (CAS No. 56252-56-3), methylaluminoxane, isobutylated methylaluminoxane, isobutylaluminoxane, tetraisobutyldialuminoxane or bis(diisobutyl)aluminum oxide.
Different magnesium compounds or aluminum compounds may also be used together.
The polymerization of the styrene is particularly preferably carried out in the presence of a trialkylaluminum and/or dialkylmagnesium compound.
The molar ratios of the lithium compounds to the magnesium or aluminum compound can be varied within wide ranges. It depends in particular on the desired molecular weight, the desired polymerization rate and polymerization temperature and the type and amount of monomers. Expediently, the molar ratio of magnesium or aluminum to alkali metal is from 0.2:1 to 5:1. If magnesium and aluminum compounds are used together, the polymerization is carried out at a molar ratio of the sum of magnesium and aluminum to lithium of from 0.2:1 to 5:1.
The amount of sterically hindered phenol or amine added depends on, inter alia, the desired retardant effect. If only monomers stabilized with sterically hindered phenols or amines are used, the polymerization rate is essentially influenced by the magnesium or aluminum compounds. However, the sterically hindered phenol or amine is preferably used in larger amounts. This leads to better control of reactivity and higher thermal stability of the living polymer chains. If the phenol is used in a substoichiometric amount relative to the magnesium compound, the excess magnesium can itself act as a chain initiator. The polymerization is preferably carried out at a molar ratio of magnesium or aluminum (or the sum of magnesium and aluminum if magnesium and aluminum compounds are used) to phenol or amine (or the sum of phenol and amine) of from 1.1 to 100, particularly preferably from 1.5 to 10.
Particularly preferably the novel process can be used for the preparation of high-impact polystyrenes. For this purpose, the vinylaromatic monomer is polymerized in the presence of a copolymer of vinylaromatic monomers and dienes as rubber.
The rubber used is particularly preferably an asymmetrical styrene-butadiene-styrene three-block copolymer S1-B-S2, where S1 is a styrene block having a weight average molecular weight Mw of from 5000 to 100,000 g/mol, preferably from 10,000 to 40,000, g/mol, B is a butadiene block having a weight average molecular weight Mw of from 12,000 to 500,000, preferably from 70,000 to 250,000, g/mol and S2 is a styrene block having a weight average molecular weight Mw of from 30,000 to 300,000, preferably from 50,000 to 200,000, g/mol.
The residual butadiene content of the styrene/butadiene block copolymers used and of the homopolybutadiene should be less than 200 ppm, preferably less than 50 ppm, in particular less than 5 ppm.
The rubber content is expediently from 2 to 25% by weight, based on the high-impact polystyrene.
The conversion, based on styrene of the hard matrix, is as a rule more than 90%, preferably more than 99%. The process can in principal also lead to complete conversion.
The high-impact polystyrene can also be prepared by first preparing a lithium-terminated polydiene and then simultaneously adding vinylaromatic monomers and the magnesium or aluminum compound. This results in the formation of a polydiene/vinylaromatic monomer block copolymer and a hard matrix comprising vinylaroamtic monomers. The block copolymer can also first be polymerized by adding vinylaromatic monomer and then terminated by an H-acidic compound, for example an alcohol. A lithium compound, for example a lithium alcoholate, forms. The polymerization of the hard matrix is then carried out in the presence of the magnesium and/or aluminum compound. In the presence of the lithium compound formed, further initiation with a lithium organyl can be dispensed with.
The growing polymer chains in the novel process have high thermal stability. Consequently, polymerization at high monomer concentrations and high temperatures is permitted. The sterically hindered phenol used is formed again on acidification of the polymer solution and simultaneously serves as a stabilizer for the polymer.
Further conventional assistants such as stabilizers, lubricants, flameproofing agents, antielectrostatic agents, etc., can be added to the novel polymers.